1. Field of the Invention
The present invention generally relates to a beam-to-column joint to be utilized in the frame construction of buildings or other structures that are subject to seismic loads. In particular, the beam-to-column joint is a pin-fuse joint designed to lengthen dynamic periods and effectively reduce the forces that must be resisted within the frame so that the frame can withstand seismic activity without sustaining significant damage.
2. Description of the Related Art
Structures have been constructed, and are being constructed daily, in areas subject to extreme seismic activity. Special considerations must be given to the design of such structures. In additional to normal loading conditions, the frames of these structures must be designed not only to accommodate normal loading conditions, but also those loading conditions that are unique to seismic activity. For example, frame joints are typically subject to cyclic motions during seismic events. To withstand such loading conditions, structures subject to seismic activity must behave with ductility to allow for the dissipation of energy under these extreme loads.
In the past, most frame joints subject to seismic loads have been designed with the beam flanges connected directly to the column flanges via full penetration welds and with the beam webs either bolted or welded to columns. In recent seismic events, including the Northridge Earthquake in Northridge, Calif., moment-resisting frames of these types successfully prevented buildings from collapsing due to applied seismic loads. While these moment-resisting frames have proven successful in preventing buildings from collapsing, the frames have not done so without sustaining significant damage. After being subject to seismic loads, most of these types of moment-resisting frames have exhibited local failures of connections due to poor joint ductility. Such joint failures have raised significant concerns about the structural integrity and the economic performance of currently employed moment-resisting frames after being subject to an earthquake.
Since the Northridge Earthquake, extensive research of beam-to-column moment connections has been performed to improve the ductility of joints subject to seismic loading conditions. This research has lead to the development of several modified joint connections, one of which is the reduced beam section connection (xe2x80x9cRBSxe2x80x9d) or xe2x80x9cDogbone.xe2x80x9d Another is a slotted web connection (xe2x80x9cSSDAxe2x80x9d) developed by Seismic Structural Design Associates, Inc. While these modified joints have been successful in increasing the ductility of the structure, these modified joints must still behave inelastically to withstand extreme seismic loading. It is this inelasticity, however, that causes joint failure and in many cases cause the joint to sustain significant damage. Although the amount of dissipated energy is increased by increasing the ductility, because the joints still perform inelastically, the currently designed joints still tend to become plastic or yield when subjected to extreme seismic loading.
Although current joint designs may be able to withstand a seismic event, the damage caused by the joints"" inability to function elastically, raises series questions about whether currently designed structures can remain in service after enduring seismic events. A need therefore exists for a moment resisting frame that can withstand a seismic event without experiencing significant joint failure so that the integrity of the structure remains relatively undisturbed even after being subject to seismic activity.
The present invention is a xe2x80x9cpin-fusexe2x80x9d joint that is generally utilized in a beam-to-column joint assembly. The pin fuse joint is designed to be incorporated into the frame of a building or other structure subject to seismic activity and to improve a structure""s dynamic characteristics by allowing the joint to slip under extreme loads. This slippage changes the structures dynamic characteristics by lengthening the structures fundamental period and essentially softening the structure, allowing the structure to exhibit elastic properties during seismic events. By utilizing the pin-fuse joint, it is generally not necessary to use frame members as large as typically used for a similar sized structure to withstand an extreme seismic event, thereby overall building costs can also be reduced through the use of the pin-fuse joint of the present invention.
The pin-fuse joint is generally employed near the end of a beam, where the beam attaches to the column. Rather than attaching directly to the column, a plate assembly is welded to the column and extends therefrom for attachment to the beam. It is the attachment of the plate assembly to the beam that creates the xe2x80x9cpin-fusexe2x80x9d joint. To create the joint, the plate assembly and the end of the beam are designed to mate and be held together by a pipe/pin assembly extending through the web of the beam and through connection plates that extend outward from the plate assembly. Additionally, the end of the plate assembly has a curved flange end connector which abuts against an opposing curved flange end connector located at the end of the beam. The curved flange end connectors of the plate assembly and the beam are then secured together by torqued high-strength steel bolts. These bolted connections allow for the curved flange end connector of the beam to slip relative to the curved flange end connector or the plate assembly when subject to extreme seismic loads without a significant loss in clamping force. Movement in the joint is further restricted by treating the faying surfaces of the curved flange end connectors of the plate assembly and the beam with brass. The brass shims used within the connection possesses a well-defined load-displacement behavior and excellent cyclic attributes.
The friction developed from the clamping force from the bolted flange end connection and the brass shims against the steel surface of the curved flange end connectors prevent the joint from slipping under most service loading conditions, such as those imposed by wind, gravity, and moderate seismic events. High-strength bolts are torqued to provide a slip resistant connection by developing friction between the connected surfaces. However, under extreme seismic loading conditions, the level of force applied to the curved flange ends exceeds the product of the coefficient of friction times the normal bolt clamping force, which cause the joint to rotate while maintaining connectivity.
The rotation of the joint during seismic events provides for the transfer of shear forces and bending moment from the structural steel beams to the structural steel columns. This rotation dissipates energy, which is also known as xe2x80x9cfusing.xe2x80x9d It is this energy dissipation that reduces potential damage to the structure due to seismic activity.
Although the joints of the present invention will slip under extreme seismic loads to dissipate the energy, the joints will, however, remain elastic due to their construction. Furthermore, no part of the joint becomes plastic or yields when subjected to the loading and the slip. This allows a moment-resisting frame utilizing the joint construction of the present invention to remain in services after enduring a seismic event and conceivably resist further seismic activity.